text:
Why do we lean the mixture when the air become less dense?
In a carbureted engine at higher altitudes, less dense air means there is less air entering the engine. Wouldn't we want the mixture to be full rich to allow the maximum air into the engine?
I understand that the engine will become rough if we don't lean the engine. I'm asking what is going on? It seems like a decrease in air means we would need to open the mixture not close it.

Full rich = maximum fuel? Why would you want to add more fuel to compensate for less air?

Mixture controls fuel. You may be mixing things up here.

@egid Throttle controls fuel, mixture controls how much of the fuel-air mixture enters the cylinder

Not true. Mixture controls the fuel-air ratio, by controlling how much fuel is added to the air. Throttle controls how much of the air-fuel mixture enters the cylinders. I promise.

Egid is right. Mixture controls fuel-air ratio and throttle controls the amount of the mixture. However the optimal fuel-air ratio, by mass, is simply given by the chemistry and shouldn't (and doesn't!) depend on pressure/altitude. So one would expect not having to touch the mixture control. Yet, one has to and that still needs to be explained (and none of the existing answers explain).

@egid Thanks. Turns out you're right. I found this video about carburetors https://www.youtube.com/watch?v=oIU-lGc3DL4. It says the throttle is the second valve which correlates with what you said

I'm a CFI, I'd hope I'm right ;)

The decrease in air means less oxygen. If the fuel is burnt at "rich" there would not be enough oxygen in the air to burn some of the fuel, thus reducing efficiency, increasing the risk of carbon monoxide, and adding grime to the exhaust system. That is why "lean" is used for high altitudes, to burn efficiently and safely.

This does not explain anything. Mixture is the fuel-air ratio. When you reduce air by closing throttle, fuel is reduced accordingly to keep the fuel/air ratio the same. So what needs to be explained is why at lower pressure the same setting of mixture results in higher fuel/air ratio (richer). And for carburettors and injection the reason may not actually be the same.

@JanHudec engine mixture controls aren't based on mass, they're based on air volume. As altitude increases, air pressure (air mass) decreases for a given volume. This causes the fuel mass to gradually increase relative to the air mass. Leaning the mixture is required to bring that back towards the correct ratio.

@egid, I see it isn't based on weight, at least in carburettor. But why it is based on volume would deserve explanation; I don't see that.

As an aside: this is also why you may choose to run lean while the engine is warming up on the ground - to avoid unburnt fuel fouling the plugs and exhaust.

Internal combustion gasoline engines need to have a constant air/fuel ratio of 14.7:1 for efficient combustion.
The air/fuel mixture by volume remains constant as we climb but the air/fuel mixture by weight does not. The carburetor only knows the volume of air passing through it, and because the weight of the air becomes less as we climb, the mixture becomes too rich at altitude. THAT is why we have to lean the mixture as we climb
"Leaning" the mixture mixes less fuel with the air in order to keep the air/fuel ratio at a constant 14.7:1 ratio.
Even with a leaned mixture, as the air becomes less dense, less power is produced because less air and fuel enters the engine.
Forcing additional air into the engine with a mechanical driven compressor (supercharger), or exhaust driven compressor(turbocharger), can restore normal sea level power to the engine.
At sea level, forcing additional air into the engine can also boost performance to more than is available in a "normally aspirated" engine.

So the term 'leaning' isn't really correct. It suggests the air/fuel ratio is changed, while it is in fact kept constant. Sometimes language tends to make technology more confusing than it already is...

Actually I think "leaning" it is correct: If you don't pull back on the mixture control to "Lean" the air/fuel mixture as you climb, the engine will be running too "Rich".

It depends on what you mean: I see 'leaning' as decreasing the ratio, not as altering the absolute fuel flow. But that last thing is what actually happens. Same confusion lurks when we get to talk about throttling. So let's stay on topic and not let language get in the way too much ;)

My original comment was meant to help the OP understand your excellent original answer and avoid Language Induced Confusion)

@MikeSowsun why does a richer mixture create more efficiency and why does a leaner mixture create better performance?

Try coffee to understand. You only like it with sugar, but it should be just right, not too sweet. Now imagine there's just a little coffee left in the can (high altitude). You can't get a full cup. So there's less coffee (air), you put in less sugar (fuel). You don't 'lean' your coffee, you get it just the way you always drink it. Afterwards you'll feel less awake though (engine performance).

But why does the mixture become richer as the aircraft climbs? The point of carburettor, or air weight in injector system, is to maintain the air/fuel ratio constant, so what needs to be explained is why it fails to do it without assistance as the pressure changes.

The air/fuel mixture by volume remains constant as we climb but the air/fuel mixture by weight does not.  The carburetor only knows the volume of air passing through it, and because the weight of the air becomes less as we climb, the mixture becomes too rich at altitude. THAT is why we have to lean the mixture as we climb.

Yes, weight. It's probably worth editing the answer to clarify this.

Good idea. I did edit my answer.

@MikeSowsun To the best of my knowledge, aviation piston engines are not operated at the stoichiometric ratio (14.7:1). Doing so can result in an engine overheat situation, which may lead to problems like pre-ignition and detonation. During conditions of high power and low forward speed (takeoff and climb-out), a rich mixture is used to keep temperatures under control (the excess fuel absorbs latent heat as it changes state from liquid to gas, and in the process, it cools down the intake change). Ratios of around 10:1 may have to be used under such conditions.

Under the conditions of high forward speed with medium to low power (cruise and descent), a lean mixture is used to improve fuel economy. The excess air in the charge now provides some cooling - not as much as the fuel does in a rich mixture, but usually sufficient to prevent overheating. The only time when it is relatively safe to use the stoichiometric ratio is when the engine is at a low power setting. (Though, some modern aviation engines may be able to handle the stoichiometric ratio at cruise power without overheating)

Not only is a rich fuel mixture inefficient, if it is too rich there can be zero combustion.
A long time ago in auto shop, the shop teacher demonstrated the effect of too much gas and not enough oxygen. He took a old coffee can and sprayed some gas in it, then covered it with the plastic lid, that had a small hole in it. Placed a match at the hole and the resulting small fire was about the size of a small candle flame. After some time (about a minute), the fire had burned enough fuel, the remaining fuel-air mixture was at the correct ratio, and they burned with a rapid little whoosh.

Welcome! That's indeed a good explanation of mixture ratio, but it doesn't yet answer the question. Consider editing your post to make it a full answer. See [answer] for additional details.

Airplanes with a Mixture control have internal combustion engines (like automobiles and lawnmowers, but not like jets).  An internal combustion engine runs on fuel (avgas, petrol, butane, alcohol, whatever hydrocarbon you pick...) mixed with air.
The ratio that the mixture of fuel and air is set to is important for several reasons:  cost of fuel, engine cooling, avoiding carbon buildup in the engine, how smooth the engine runs, how much power the engine produces, etc.
The ratio is based on the number of molecules of fuel and the number of molecules of oxygen.  Air is approximately 20% oxygen no matter if at sea level or 30,000 feet.
But at sea level the air is compressed more than at 30,000 feet.  As the airplane, or you, go up in altitude the air gets "thinner", or less dense.  Less dense means that the air is not as compressed.  For some volume of air, 1 cubic foot for example, higher pressure air will have more air molecules than lower pressure air.  Since air is 20% oxygen, fewer air molecules also means fewer oxygen molecules.
The engine performs best when the air/fuel mixture ratio stays "in balance", or as close to ideal as possible.  As the engine is running is pulls air into the engine, but the amount of air is based on the volume of air, not the number of molecules of air.  As the pressure decreases, so do the number of molecules.  As the aircraft climbs into thinner and thinner air the mixture must be "leaned out", where the volume of fuel and the fuel molecules is reduced, to compensate for the fewer number of oxygen molecules being pulled into the engine.
As the aircraft descends into "thicker" air, the number of oxygen molecules increase again, so the mixture must again be adjusted by increasing the number of fuel molecules to keep the air molecules and fuel molecules at the correct ratio.


text:
What is Special VFR? Why would a pilot request it?
I have heard about Special VFR many times but never understood this concept. What is it and when is it used?

Heeeey. Stop closing questions I can answer :D @fooot

@SentryRaven go answer one of the duplicate targets instead :)

To be honest, I don't even think this is a duplicate. The other questions are already a bit more specific coming from someone who understands what SVFR in general is, but wants clarification on finer points. This question here asks what SVFR is for to begin with...

Special VFR is used to leave or enter a control zone coming from or continuing the flight in airspace G (Golf), when the weather minima for VFR flight are not met within the control zone, but would be met in airspace G.
Example: Airspace G (Golf) requires a visibility of 1.5km and aircraft need to remain clear of clouds. Airspace D (Delta) requires a visibility of 5km and 1000ft vertical and 1.5km horizontal separation to clouds.
If the weather report would show a visibility of 4km/4000m and a cloud base of 1400ft, the flight would be allowed in airspace G but not in airspace D.
Requesting Special VFR within a control zone will drop the VFR weather requirements to the values required for airspace G, so flight may enter or leave the controlzone. The lower visibility requirements are compensated for in the control zone by stricter separation requirements between aircraft. SVFR is separated to SVFR and IFR traffic.

For comparison: In the US class D VFR minimums are 3 sm vis and 500/1000/2000 feet below/above/horizontal clearance from clouds.  The difference from class G minimums are only during the day, when class G has 1 sm visibility required and clear of clouds below 1200 AGL.

The OP didn't ask if they wanted to know FAA or ICAO, so I threw ICAO at them. :D @casey

@SentryRaven Oh, ICAO....

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text:
How does a pilot file a PIREP?
For what reasons would a pilot want to file a PIREP?
If a pilot wanted to file a PIREP, how would he/she go about filing one?

I'll tackle the "How" first - you call up Flight Service (either on the radio or on the phone after you land) and tell them you have a pilot report.  There's a standard format for pilot reports, and if you give all the information in the order it appears on that form it's faster for you and the person taking the report.
If you're filing a pilot report on the ground for conditions you encountered enroute you need to specify the (Zulu) time of your observation (or tell the briefer "20 minutes ago"). In the air it's generally assumed your report is for "now".

Pilots are requested to provide pilot reports for significant deviations from the forecast weather, or for hazardous conditions. Basically we provide them as a courtesy and service to our fellow pilots (like flashing your lights at other cars on the highway if you know there's a speed trap).
Some pilot reports you may want to file in flight are if the Winds Aloft differ from the forecast. If the forecast told you you should have a 10 knot tailwind and you actually have a 5 knot headwind that's pretty significant, and if you tell Flight Service they will pass it along to the weather forecasters and they'll factor it in to their next Winds Aloft calculation. It will also appear in the next pilot's briefing so they'll know the winds in the forecast aren't correct.
Another example is turbulence: If you're encountering continuous moderate chop at 7,000 feet but smooth air at 9,000 feet providing that information could be useful to the next pilot flying through the same area.
A classic example of a PIREP you file after you're on the ground is icing: If there was no icing in the forecast but you've encountered some you should report these conditions. If you're under ATC control you'll obviously tell them, but when you're on the ground you should call Flight Service and file a formal pilot report. Again, the information will be passed to the weather forecasters and the icing AIRMET will be updated, and your PIREP will be passed along to other folks in their briefings to keep them out of the ice.

Note that you can also submit PIREPS on http://aviationweather.gov but I've never used that option: There are more hoops to jump through than I'd care to deal with, and IMHO it's faster to just talk to Flight Service.

I usually just make them directly with approach or center if the frequency is slow, and I usually only include the relevant items. "Seattle Center, Cessna 123 with a quick pirep." "Go ahead." "4500 over LOFAL, winds 320 at 15, overcast 5000." These almost always end up in the system.

@egid yeah that also works (and if you're IFR and the frequency is slow its better than leaving the frequency). Nobody ever talks to the Flight Service guys around here though, it's like visitng Grandpa Simpson in the home, they're so happy to have someone to talk to :)

like flashing your lights at other cars on the highway if you know there's a speed trap - It's so good to know I'm not the last one on the planet that does that! Nobody in Indiana seems to have a clue about it... :/.  Back to your regularly scheduled discussion of aircraft.

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text:
How does one determine if an airplane meets airworthiness directives?
There seems to be no way to determine if airworthiness directives for your airplane have been complied with. Does anyone know? I found this FAA website but you can type in a type of aircraft and nothing comes up. Also does anyone know where I can find safety bulletins for an aircraft?

Compliance with Airworthiness Directives (ADs) involves an appropriate logbook entry. Examine the aircraft logbooks, or have someone competent in the subject matter—an aircraft mechanic, for example—to determine if applicable ADs have been complied with.
Each AD requires different compliance measures, ranging from a one-time inspection or parts replacement to recurring inspections. In each case, a logbook entry should be made to document compliance. Apart from the logbook entry, the aircraft or components concerned can often be inspected to confirm compliance, such as checking for a marked part ot that a placard installed. The details of what such an inspection could confirm would vary depending on what the AD requires.
In some cases, such as AD 68-17-04, which deals with Cessna stall warning systems, the AD calls for a one-time inspection followed by installation of a placard. The placard calls for the pilot to test the stall-warning system as part of pre-flight. In such a case, compliance with the AD extends beyond the actions of maintenance personnel, and will include pilot compliance with aircraft limitations—the placard.

Now, you seem to be asking three questions, beyond the title question of How does one determine if an airplane meets airworthiness directives? Let me paraphrase these questions:

How does one determine if ADs have been complied with?
How does one find ADs that apply to an aircraft?
How does one find "safety bulletins" for an aircraft?

The first question I have answered above.
The second question is primarily the purview of aircraft mechanics, especially IAs where applicable. The site you linked is a good way to find applicable ADs. It sounds like you may have run into some difficulty in getting the site to work correctly. A search for "Cessna 172" returns 21 ADs. Since ADs list aircraft or component applicability by make, model, and serial number range, among other criteria, each AD will need to be examined to determine if it is applicable to any given aircraft.
The third question is more difficult. Airframe, powerplant, and parts manufacturers may issue Service Bulletins and disseminate them at their choosing. Some manufacturers are easier to deal with in this regard than others. An exhaustive answer on this topic is beyond what is appropriate here, in part because it is constantly changing. If in doubt, contact the manufacturer for what you need.

Then again, not all logbook entries are correct! (Omega Air found this out the hard way by losing N707AR to an AD issue that was documented as fixed but wasn't permanently closed out properly)

In fact, should the question "how do you verify that an AD logbook entry is reflective of the status of the actual aircraft?" be answered here, or in a question of its own?

@UnrecognizedFallingObject Usually when purchasing an aircraft you would verify that these logbook entries are accurate with the assistance of an A&P. If you are renting the aircraft then it becomes a little more difficult. You can talk to the A&P that performed the AD's, and its well within your rights to examine the airframe/engine logbooks prior to you renting the aircraft.

@RonBeyer -- perhaps I could ask it as "how can an A&P verify that an AD logbook entry correctly reflects the modification status of the aircraft?"

@UnrecognizedFallingObject If that question were asked, the only viable answer I can think of would be, "by inspecting the logbook and aircraft". Beyond that, I would be inclined to VTC the question as too broad since the details would depend entirely on the requirement of the AD(s) in question.

@UnrecognizedFallingObject actual verification of AD or Service Bulletin compliance by an A&P often means re-doing the required work. Some are easy to verify (there's a Service Bulletin on my plane that requires drilling drain holes in the bottom of the stabilator, you can look at the control surface to verify the holes are there.) and others require a lot of work (Piper SB 1006 is CASA AD 2005-0032, the only way to verify compliance is to remove the fuel tanks and do the inspection).

How does one test the stall warming pre-flight?

@TomMcW Good question! I would recommend asking it as a new question; a full answer would warrant a new Q&A.

@JonathanWalters done

Another big area are Appliance AD's.  They cover parts which may or may not be installed on a wide variety of aircraft, and simply searching for the aircraft type will not pick them up.  It's up to each owner/operator to ensure compliance with them all though!

In your question you started out asking about airworthiness directives (ADs) and then switched to service bulletins (SBs). They are two different things with different implications depending on which rules you are flying under. For general aviation aircraft that are flown under Part 91 of the FARs you must comply with ADs in order for your aircraft to be airworthy. i.e. if you haven’t complied with the AD (and that includes documentation) then in the eyes of the FAA and your insurance company your aircraft is not airworthy. For Part 91 operators, Service Bulletins are advisory—even though many of them say in their text that they are mandatory.
As Jonathan mentioned, it is a tremendous amount of work to read through all of the ADs that might be applicable to an aircraft to determine whether it applies to that specific aircraft. Several companies make software that IAs can use to simplify the process. (My IAs use Tdata.) The IA puts the serial number of the aircraft, engine, alternator, magnetos, oil cooler, etc. into the software and the software goes through the tedious process of determining which ADs apply to that particular aircraft. The owner or IA can then go through the logbooks and determine which ADs have been complies with. The software prints out a record of compliance that the IA signs and it is incorporated into the logbooks. This process is required at every annual inspection in order for the inspection to be signed off on.
Service Bulletins on the other hand are not issued by the FAA but instead are issued by the manufacturer of the part in question. It is up to the IA and the operator whether or not to comply with them. For example, SERVICE BULLETIN No. TP-14 Rev. 3 claims that replacement of Parker Hannifin vacuum pumps is mandatory. It is not. Only the FAA can mandate replacement of parts and then only by issuing an AD.
In between ADs and SBs are Special Airworthiness Information Bulletins (SAIB) ”They are an information tool that alerts, educates, and makes recommendations to the aviation community. SAIBs contain non-regulatory information and guidance that does not meet the criteria for an Airworthiness Directive (AD).” As an owner/operator you should be aware of them and determine whether compliance is warranted. For example, SAIB: CE-14-23 was issued for Piper aircraft to alert A&Ps to a potential unsafe condition on inlet hoses. The inlet hoses for the carb look a lot like regular scat tubing that is used elsewhere in the engine. Unfortunately, scat tubing isn’t as rigid as inlet hose and it can collapse and cause the engine to quit from lack of air. Piper noticed that they hadn't sold any inlet hoses in a long time and suspected that mechanics were using the wrong hose. My A&P checked, and sure enough I had the wrong hoses. Someone in the past, either through ignorance or cheapness, had replaced the 350 dollar hoses with 10 dollar hoses.
My answer is only for Part 91 Operators. Part 135 Charter Operations are usually required to comply with service bulletins as well as ADs—depending on how their certificate is written. Air Carriers and Freight operations have rules that I am not familiar with, so someone else will have to chime in on those.


text:
How long is a winds and temperatures aloft forecast valid for?
How long is a winds and temperatures aloft forecast valid for and how often is it published?
Also, is there anywhere which says the times of publication and valid through times for all of the aviation weather charts? My google fu isn't helping.

Advisory circular 00-45 "Aviation Weather Services" documents the winds and temperature aloft (FB) product on page 7-39.  Table 7-11 lists the standard forecast periods for these products.

From this table you can see that the 6-hour forecasts are usable from the time they are issued until 3 hours after the valid time.  The 12 hour forecasts are usable from -3 to +6 hours of the valid time and 24 hour forecasts are usable from -6 to +6 hours of the valid time. The products are issued 4 times daily.
While these are standard times, you should consult the specific information available in the product header that has this information.  For example:
DATA BASED ON 010000Z
VALID 010600Z FOR USE 0500-0900Z. TEMPS NEG ABV 24000
FT 3000 6000 9000 12000 18000 24000 30000 34000 39000
MKC 9900 1709+06 2018+00 2130-06 2242-18 2361-30 247242 258848 550252

This says the product is valid at 06Z and for use from 05Z to 09Z and the data is based upon the 00Z forecast model run.

According to NOAA:
(https://www.aviationweather.gov/products/nws/info)
Sample winds aloft text message:

DATA BASED ON 010000Z VALID 010600Z   FOR USE 0500-0900Z. TEMPS NEG
  ABV 24000 FT  3000    6000   9000    12000   18000   24000  30000
  34000  39000 MKC 2426 2726-09 2826-14 2930-21 2744-32 2751-41 275550
  276050 276547

The portion that indicates valid times:

VALID 010600Z FOR USE 0500-0900Z.

The valid time of the forecast is the first day of the month at 0600 UTC. The forecast winds and temperature are to be used between 0500 and 0900 UTC.
The frequency of publication is described explicitly here:
(https://en.wikipedia.org/wiki/Winds_aloft)

This forecast is now made four times a day based on 0000Z, 0600Z, 1200Z, and 1800Z

I found this helpful - clarified what valid time is vs for use period (from: http://www.flyingactivity.com/mod/fileman/files/FBDoc.PDF)

The FB product is available four times a day based on the synoptic
times of 00Z, 06Z, 12Z, and 18Z. Those are the nominal times at which
new model runs are made. The results are made available from one and
one-half hours to two hours after the synoptic times. Each run
generates three forecasts for three different time periods over the
next 24 hours. Each model run is made for three specific times, and
the results are strictly valid only for those times. These valid times
are six, twelve, and twenty-four hours after the model run synoptic
time. However, the data are declared to be useful for a certain time
period about the specific valid times. So each forecast period has a
valid time and a “for use” time period. These are described in the
following table (from
http://aviationweather.gov/products/nws/winds/info.php):

There is also an UPPER WIND AND TEMPERATURE product issued by WAFC London.  These are issued in 3-hr increments and valid -/+ 1.5 hrs.  So a 1200Z chart would be valid 1030Z - 1330Z.  In raw form they usually have a product code associated with them.  Example: PWBE85.

Every forecast has a valid time, and in fact valid times are different for the same type of forecast in different locales , for example TAFs are valid for either 4 or 6 hours.
My answer, therefore, is that the valid time for ANY forecast is whatever the forecast itself says.

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text:
In a pattern, when do you begin your descent from TPA if following another aircraft?
I had always been taught to begin a descent from TPA abeam the touchdown point, even if tower tells me to follow another aircraft. Yesterday I flew with an instructor who did not want me to descend from TPA until abeam the aircraft I was following. I understand that I begin my base turn when the aircraft I am following is abeam my wing. I did not know to also wait until then to descend from TPA. It does make more sense to stay at TPA until this point but I can't find anything saying one way or the other.

Were you also extending your downwind, or did you turn in at the normal time? If you are extending your downwind then it makes sense to wait, if you are turning in normally this may mean that you are high on final. I assume that if you are "abeam the other aircraft" that you are extending?

Can you please clarify for me what TPA means? I don't think that's a term used where I'm from

Traffic pattern altitude

@rbp thanks. I've only ever heard it referred to as 'circuit height' here in Australia.

@Ben we Americans love our TLAs.

@RonBeyer We were extending the downwind, yes.

@jskypilot In that case you are going to have to wait, otherwise you are going to be very low on final and dragging it in. Depending on how far the controller puts you out, you may not even start your descent until after you've turned final. I'm not sure why your first instructor would have you descend immediately, especially if the controller says "extend downwind, I'll call base", which means you may be heading out for a while...

Every CFI I have ever flown with has a different procedure for when to descend. Its annoying. Think of it as descending to the runway when you are in a position to land. If you are not following another aircraft, then you can begin your descent abeam the threshold. If you are following another aircraft, then you should wait to descend until you are in a position to land as #2, which could be after you've already turned base or (long) final.


text:
An instrument applicant must have 40 hours of actual or simulated time. Can some of this be flown without an instructor?
61.65 says an instrument applicant must have 40 hours of actual or simulated instrument time, 15 of which must be from an authorized instructor. What about the remaining 25 hours? Someone without an instrument rating can't file and go fly an IFR flight, right?

You can't fly on an instrument flight plan, but you can practice IFR procedures using a safety pilot, which does not have to be a CFI.

@Pondlife why is it bad if there is a duplicate answer? The questions are worded much differently, in my opinion my question is more clear than the question you linked to. Sometimes there may be a duplicate answer for two different questions. I think that is ok as long as it is not the exact same question which was already answered.

@jskypilot You can read more here but basically, most SE sites are trying to find the best answer to a given question, and dupes can make finding that answer confusing and difficult. Even if this question is closed as a dupe, it will still be here and searchable, so people can still use it as a stepping stone to the definitive answer.

The 25 hours that are not required to be "instrument instruction" may be flown under the hood with a safety pilot. This is the usual way to accumulate some of the simulated instrument time.
I believe this time may also be flown & logged in actual IMC with an instrument rated pilot acting as PIC (their name on the flight plan, etc.) and the instrument student acting as sole manipulator of the controls, but you may prefer to conduct your actual IMC practice with an instructor.

Unless, the "sole manipulator of the controls" is a required flight crewmember, that time cannot be logged.  FAR 61.51  A non-instrument rated pilot cannot log instrument time in IMC without a CFI unless she/he is a required crew member.

@STWilson I'm looking at 14 CFR 61.51 and that doesn't seem to be true. 61.51(e)(1)(i) states that a private pilot may log PIC time when they are the sole manipulator of controls for an aircraft they're rated in. 61.51(g)(1) states that instrument time can only be logged in simulated or actual instrument conditions. I don't see anything in 61.51 supporting your statement.


text:
What are the differences between a slot and a slat?
If you look at image 5-18 in the PHAK Chapter 5 about leading edge lift devices you see this:

However the paragraph preceding it states

High-lift devices also can be applied to the leading edge of
  the airfoil. The most common types are fixed slots, movable
  slats, leading edge flaps, and cuffs.

So is this a typo? Should the image say 'movable slat' instead of 'movable slot'?
This leads into my main question which is, what are the differences between a slat and a slot? If I were to get asked this by an examiner what would a good response be?

I guess it should be movable slat.
A leading edge slot is basically a spanwise opening in the wing. Slats are aerodynamic surfaces in the leading edge, which when deployed, allows the wing to operate at higher angle of attack. When deployed, the slat opens up a slot between itself and the wing.

Image from simhq.com
In some aircraft, the slats are fixed, which opens up a slot between the wing and the slat. In this case, the terms slot and slat are used interchangeably.

"Leading edge slot" by Sanchom - Own work. Licensed under CC BY 3.0 via Commons.
A number of airliners use movable slats, in which case, the system is called slat, rather than slot.

"Voilure A319" by Nicourse - Own work. Licensed under CC BY-SA 3.0 via Commons.
In short, the system is pretty much the same, but is (usually) called slat in case of movable one and slot in case of fixed one.

Would it be fair to say that a slat is a movable device that creates or closes off a slot in the leading edge of the wing? It seems to me that fixed slot is simply a permanent, spanwise hole in the leading edge of the wing.

@FreeMan Yes. It would be fair. This was the meaning in which the term slot was originally used.

Are a leading edge cuff and a droop just two names for the same thing?

@TomMcW The relationship is kinda similar. The droop is usually a deployable LE device. LE cuff is a fixed LE droop and is used as a general term, which probably got its name from the shape of the sharp cut off point. NASA did a lot of research into this for spin resistance and called it drooped LE, not cuff.


text:
Given aircraft and runway characteristics, how do you compute the maximum temperature for take-off?
Consider an airport e.g. Phoenix Sky Harbor International Airport (PHX), with Maximum runway length (ft) of 11500 and Elevation (ft) of 1135. For Boeing 737 whose Maximum takeoff weight is 174,200 lbs, How do you compute the maximum/optimum temperature for takeoff

Well, there is some subjectivity here, but I consider the optimum temperature for takeoff to be 72-degrees Farenheit.
As far as the outside temperature is concerned, there is no specific optimum. In general, colder is better.

This gives an answer, but does not explain how that answer was arrived at, which seems to be what the OP is after.


text:
What are the design trade-offs between a pusher and a puller propeller?
Wich is the difference in having the propeller forward or back in an airplane.
I would like to know the technical differences like in a Piaggio 180 or in a normal Cessna.

Technical differences is very broad, you mean in structural, aerodynamic, flying characteristics? Something else?

I mean during the projecting phase..

The difference that can make to choose a solution than another

Ty right! Ty all !


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What type is this aircraft and engine at Munich airport?

Please help me to identify the plane and engine.

Lufthansa Airbus A350-941, 2x RR Trent XWB?

Welcome to aviation.SE!

The fact that it is a single deck, twin engined aircraft with that distinctive wingtip confirm that it is an Airbus A350, operated by Lufthansa. The -900 series is the only A350 in service at this time, and Lufthansa has not ordered the -1000 series.
The A350 is powered by Rolls Royce Trent XWB engines.

absolutely! Airbus A350-941 D-AIXC

@user27467 How can you tell it is (D-AIX) C?

because I know date and time of photo, and there are few A350 in LH fleet

Nice plane! I was lucky to fly it twice last week (BOS <-> MUC)


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What approvals do I need for a helicopter landing?
I have a horrible conflict; my sons are graduating on the same day at about the same time in different states. One in East Greenbush NY the other in Beverly MA.
I am interested in hiring a helicopter to transport us from NY to MA. My question is if I receive permission from the schools to land what other permits or steps would I have to take?

You have deeper pockets than me @Paula, that will cost you serious bucks. In any case it's hard to say exactly, it depends on the local laws. It would be a lot cheaper and faster to get a private fixed wing airplane from a local airfield like Alexander Farm to Beverly airport with car transfers in between, each one's just a couple of miles from your start and end points.

See this question. But if you're chartering a helicopter - or any other type of aircraft - then dealing with landing permissions and other paperwork is the operator's job, not yours.

@GdD the distance between East Greenbush and Beverly is approx. 125 NM. A helicopter transfer will take approximately 1 hours 40 minutes, assuming ~100 knots cruising speed plus a bit for start-up and transfer to the sports field / helipad. A fixed wing may be faster, but going to/from a local airport will take approximately 20-30 minutes, on each side. The flight itself will be faster than the helicopter flight, but when you add start-up / taxi time on each end, I'd be surprised if it is still quicker overall. I reckon you need a cruise speed exceeding 160 knots to make up for the lost time.

20-30 minutes to go 2-3 miles on each end @DeltaLima? There are small airfields in very close proximity on either side. Even if she uses a helicopter they're much more likely to transit between those 2 points as getting permission to land at a school is unlikely.

@GdD, Google gives me 11 minutes by car for the first transfer, 19 minutes for the second. I padded both with 10 minutes extra time for margin. I did the same for the helicopter.

If you charter a helo, most likely they’ll fly you from KALB to KBVY, then provide you additional ground transportation to and from these airports.  As to landing directly onto school property, that would probably be at the discretion of the school itself; my guess is that they would frown on it unless it was an emergency, as helicopters are a noisy and somewhat dangerous nuisance to have around.

Comments are not for extended discussion; this conversation has been moved to chat.

Just as a ballpark estimate, a trip like that in a Robinson R44 would likely be your most cost-effective option, and would take approximately 2 hours flight there and 2 back. Even if you get 500 dollars per flight hour and no holdover charge (which would be a stellar deal), you're looking at a minimum of 2,000 dollars for this trip - probably more.
Now for the bad news and the real answer to your question. Assuming that there are no local airspace restrictions which would prevent you from landing at these schools (and there probably AREN'T... I've landed at schools before for organized educational activities), FAA regulations say that you must have the "land owner's" permission to land at each site. That means it's not enough to get the Principal's sign-off. You'll likely have to get a member of the school board of each district to provide written permission or someone else who is a signatory of the property owned by the school district.
Alternatively, if there are any privately-owned, large, empty fields that are close to the school - say, across the street or something - it might be easier to get one of those individuals to give you permission, and then you could just walk across the street...


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Which moments are acting on a flying airplane?
I would like to ask some clarifying questions regarding the moments acting on a flying airplane.
My understanding is that there are following force:

Weight (pointing down and applied to the CG of the plane)
Lift from the wing (pointing up and applied to the aerodynamic center AC)
Lift from the horizontal stabilizer in the tail (this resultant force is pointing down, it is smaller than Lift_wings, and it is applied to the AC of the tail).

The airfoil, on its own, also has an aerodynamic moment (positive)
   which would make the wing rotate nose-up about the CG of the wing
   itself if the wing was placed free in the wind stream. However, the aerodynamic moment about the aerodynamic center is constant negative (nose down). Is that correct? The physical effect is the nose up intrinsic moment of the cambered wing due to the fact that the force on the bottom and the force on the top of the wing do not act along the same line of action.
For longitudinal stability, are the participating moments the moment due to Weight, the moment due to the wing lift and the moment due to the horizontal
 stabilizer that need to add to a zero resultant moment? Can the positive aerodynamic torque of the wing be ignored because much smaller in magnitude than the other torques?
In most planes, starting from the leading edge of the wing, the aerodynamic center AC of the wing is placed behind the plane's CG. How far behind CG should the wing AC be placed? The placement of the tail's AC should easily derives from the moment equilibrium equation (sum of moments equal to zero). I realized that some big airplanes have the CG behind the wing AC. In that case, the lift from the tail is force always directed upward and not downward like when the CG is in front of the wing AC.

You should consider splitting this into multiple questions.

I've been reading this e-book recently
http://www.av8n.com/how/#contents
It goes into the things you are asking in a lot of details.

The airfoil, on its own, also has an aerodynamic moment (positive) which would make the wing rotate nose-up about the CG of the wing itself if the wing was placed free in the wind stream.

Well, that depends entirely on the location of the wing's cg. The airfoil camber determines its moment, which is negative around its aerodynamic center or neutral point for positive camber. The first derivative of that moment with respect to the angle of attack is positive, so the cambered airfoil is unstable in pitch.

the aerodynamic moment about the aerodynamic center is constant negative

Yes. That is correct. When hinged at its quarter point, the wing will pitch down, and that moment is independent of the angle of attack if the aspect ratio of that wing is large enough.

The physical effect is the nose up intrinsic moment of the cambered wing due to the fact that the force on the bottom and the force on the top of the wing do not act along the same line of action.

No, not really. Aerodynamic forces are pressures acting on an area, and the force is the result of the pressure difference between both sides of the wing. There is not an isolated lower and upper side force. Again, which sign the moment of the wing has depends on your choice of reference point. You only will get a nose-up moment if your reference point is aft of the center of pressure.

For longitudinal stability, are the participating moments the moment due to Weight, the moment due to the wing lift and the moment due to the horizontal stabilizer that need to add to a zero resultant moment?

Longitudinal stability is achieved by a negative gradient of the sum of all pitching moments over angle of attack. If you balance the moments, you trim the aircraft, but that does not say anything about stability. To simplify this task, I recommend to place your point of reference into the point in which one of the forces attacks, so you can neglect this force's moment contribution. The two preferred points are the center of gravity and the neutral point of the whole aircraft, because both don't move in flight.

Can the positive aerodynamic torque of the wing be ignored because [it is] much smaller in magnitude than the other torques?

Yes, in a first-order approximation. The tail moment is an order of magnitude bigger and dominates the pitching moments. If, however, you need to calculate the required tail size in order to trim the aircraft at all speeds, flap settings and center of gravity locations, of course you need to include the wing moment into the calculation.

How far behind [the] CG should the wing AC be placed?

It is much easier to shift the cg than to shift the aerodynamic center around - that one comes with the wing planform. Also, for stability you should look at the distance between the whole aircraft aerodynamic center and the cg - it just happens that a cg near the wing's aerodynamic center provides the aircraft with decent stability. But the cg can very well be a bit behind the wing's aerodynamic center.

I realized that some big airplanes have the CG behind the wing AC. In that case, the lift from the tail is force always directed upward and not downward like when the CG is in front of the wing AC.

Now you mix up center of pressure and aerodynamic center. That is one reason why I prefer to call the aerodynamic center "neutral point". The tail lift is only upwards for a rear cg location and high angle of attack, which is quite different from "always".


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What are the advantages of 3 spool over 2 spool turbofan engines?
What are the advantages of 3 spool over 2 spool turbofan engines?

There are two advantages;

It's easier to match the compressor rotational speed with the turbine. Typically, the first stages of the compressor want to rotate slower than the turbine is required to rotate. The turbine needs to spin faster to help obtain as much energy from the airstream as possible, but initial stages of compression prefer to spin slower. With 3 spools instead of 2, less compromise is required.
A greater pressure rise per stage can be achieved. This occurs because each compressor in a 3 spool engine (fan, LPC, HPC) will have fewer stages than each compressor in a 2 spool engine (for the same overall pressure ratio). A 5 stage compressor requires less compromise than an 8 stage compressor. The front stages prefer a high airflow to avoid stall, but the back stages can't always swallow the same mass flow, and can cause a backpressure which can lead to stall. Hence, air flow is often bled off in the middle of the compressor, to avoid stall. This is air that has been partly compressed, absorbing work, but isn't available to generate thrust. Hence, it's a parasitic load. So, the compressors in a 3 spool engine are better optimised than in a 2 spool engine, because this situation isn't as severe. Hence, the designer can be a little more aggressive in the pressure rise per stage the compressor achieves, because the compressor is less prone to stall. Thus, a 3 spool engine, in theory, can achieve the same overall pressure ratio, as a 2 spool, with fewer stages. This means, a lighter weight engine.


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Can I log PIC time when training for a tailwheel or high performance endorsement?
If I hire a CFI to train me for the tailwheel endorsement (this question also applies to the high performance endorsement), can I log PIC time when flying the tailwheel aircraft, or do I have to log dual received?
This question is specifically about FAA rules related to training/logging time, but if there are different rules in different countries, feel free to answer about those too.

Yes and Yes; As long as you are rated for the category, class, and type (if required) of the aircraft that you are receiving the training in you could log both PIC and dual received.
14 CFR 61.51(e)(i) covers the logging of PIC time (which is perfectly legal in the scenario that you describe if you are rated in the aircraft):

§61.51   Pilot logbooks.
...
(e) Logging pilot-in-command flight time. (1) A sport, recreational,
  private, commercial, or airline transport pilot may log pilot in
  command flight time for flights-
(i) When the pilot is the sole manipulator of the controls of an
  aircraft for which the pilot is rated, or has sport pilot privileges
  for that category and class of aircraft, if the aircraft class rating
  is appropriate;

Training time is logged per 14 CFR 61.51(h) and you can log that too:

(h) Logging training time. (1) A person may log training time when
  that person receives training from an authorized instructor in an
  aircraft, flight simulator, or flight training device.

That being said, you may not act as the actual PIC however:

§61.31   Type rating requirements, additional training, and
  authorization requirements.
...
(i) Additional training required for operating tailwheel airplanes.
  (1) Except as provided in paragraph (i)(2) of this section, no person
  may act as pilot in command of a tailwheel airplane unless that person
  has received and logged flight training from an authorized instructor
  in a tailwheel airplane and received an endorsement in the person's
  logbook from an authorized instructor who found the person proficient
  in the operation of a tailwheel airplane. The flight training must
  include at least the following maneuvers and procedures:
...

Notice that the regulations governing acting as PIC are different than those governing logging of PIC time.  This was done intentionally because there were so many pilots that were hired as SIC's at airlines with little to no PIC time and flew for years. When it was time for them to upgrade they didn't have enough PIC time even though they had lots of experience and could easily upgrade safely.

Yes, you can still log PIC time if you are appropriately rated for that aircraft, aside from the endorsement.
For example, if you are a Private Pilot, Airplane Single-Engine Land working on an endorsement in a Piper J-3 Cub, you would be able to log PIC time during your tailwheel training because the Cub is a single-engine land airplane.
The same goes for a MEL pilot adding tailwheel in a Twin Beech, which is an MEL airplane.
If you held only an Airplane Single-Engine Sea certificate, you could not log PIC time until you added on an ASEL rating.
14 CFR 61.51

(e) Logging pilot-in-command flight time.

(1) A sport, recreational, private, commercial, or airline transport pilot may log pilot in command flight time for flights-

(i) When the pilot is the sole manipulator of the controls of an aircraft for which the pilot is rated, or has sport pilot privileges for that category and class of aircraft, if the aircraft class rating is appropriate;


text:
What is the difference between a partial stall and a full stall?
I have this study question on the FIA written:
Which statement is true concerning the aerodynamic conditions which occur during a spin entry?
Correct Answer: After a full stall, the wing that drops continues in a stalled condition while the rising wing regains and continues to produce some lift, causing the rotation.
Incorrect Option: After a partial stall, the wing that drops remains in a stalled condition while the rising wing regains and continues to produce lift, causing the rotation.
Because the only difference between the two answer options is 'full' and 'partial' I was wondering what are the differences between a full stall and a partial stall?

For GA aircraft with straight, rectangular wings, the stall usually starts at the root and progresses towards the tip. So, at some point, only a portion of the wing will be stalled. This is sometimes referred to as partial stall, though it is not a common term.

Image from boldmethod.com
This is usually achieved by having a washout or a wing cuff. In case of root-first stall, the wing tips are still producing lift, helping to prevent rolling motion (which may occur if the whole wings stalls at once), which in turn prevents the aircraft from entering a spin.

My lecturer told us that it's purposely designed so the stall starts at the root (so the ailerons don't stall first and the pilot still has some control).

@TaherElhouderi In a manner, yes. This is one of the advantages of unswept wing. Note that some wings have devices to prevent the spreading of stall (like wing cuff etc) once it has started in the root.

Sounds like a poorly worded question on the FAA's part.  I don't think there is a useful definition of "partial" stall.  Most FAA questions come from FAA references like the Pilot's Handbook of Aeronautical Knowledge, so you could check there.  Beware that that book and other FAA publications are full of errors.
Vaguely related: If the airplane is rotating (rolling and yawing) but the airspeed is increasing rapidly, then that is a spiral dive, not a spin.

Why aren't those errors fixed?


text:
What free online educational resources should pilots in training be aware of?
Obviously this website is great as is boldmethod and several of the aviation youtube channels. Also the FAA wings website is great as is all the FAA handbooks and advisory circulars. I also like to use the ASA study guides for the oral exams. Are there any other high quality sources of information that pilots should be taking advantage of, preferably free or low cost?

Which type of license? Which type of aircraft? Which country?...

This ought to be a community wiki.

Is this close enough? Book recommendations for learning how to fly?

This website

The Airplane Owners And Pilots Association has lots of great articles on both training and new legislation that comes down the pike. The new information is really nice as it lets you know what may be or has changed since the last time you studied the books. Its worth it to join as well (its less than $50 a year and if you have just started to train they are about to bombard you with mailers anyway).
Along with publishing the FAR's the FAA publishes lots of Briefings and really great supplemental material and handbooks like this one on airspaces.
There are lots of Youtube channels out there with great information like FlightChops and the like, but note they are not in any way approved training and may not cover every aspect of what they are doing.
I like to read GeneralAviationNews.com. Its a nice source of informative articles on what is happening around general aviation, airplane ownership advice, mechanical engine advice, and all all manners of legislation that are being discussed as well as fun fly ins and things like that.


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What does this METAR remark mean? 'RMK RWY 7 ATIS G LL'
I understand the long form:
"Remarks: runway 7 Automated Terminal Information Service (ATIS) G landline"
But what I don't understand is what runway 7 has to do with the ATIS information (this particular airport has one runway, RWY 7-25). What does the 'G' mean? What does the 'landline' mean?

Can you produce the entire ATIS

KLZU 211745Z 10006KT 10SM OVC018 13/03 A3005 RMK RWY 7 ATIS G LL

I have a suspicion that you're looking at KLZU's METAR. Basically, they're inputting their manually written METAR into the system, and when they send it out they're adding the Runway in Use(RWY 7) and ATIS code(ATIS G) to their METAR, which they're really not supposed to do. I suspect that LL is the person who recorded it's operating initials at the facility, also not supposed to be transmitted as a part of the METAR.

This is KLZU, that's pretty amazing you knew that. Thanks for the info, sounds like some manual mistakes on this one.


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What is the difference between EFAS and FSS?
What services do each provide and on what frequencies?

I would go so far as to say that this is a duplicate of the question @fooot mentioned (as EFAS/Flight Watch is now consolidated with the rest of Flight Service) -- If there's something that question and this one don't cover go ahead and edit this one to clarify and we'll reopen it.

EFAS - Inflight Weather Advisory Services (aka Flight Watch) is a special frequency in the United States on 122.0.  It uses remote communication outlets (RCO) and is typically collocated with navigational equipment.
When you talk with a FSS weather briefer on EFAS you are talking with a more experience weather briefer who can provide a greater degree of accuracy with changing weather conditions.  When I took a tour or the Prescott FSS station after Lockheed Martin took over, the tour guide state the EFAS station is manned by a Level 2 weather briefer who is more knowledgeable than a Level 1 briefer who you might talk too over a regular FSS frequency.
FSS personnel can talk to pilots over RCO and the frequency is typically found along the top of the VOR dialog box on sectional charts as well as 122.2 and 122.4.  These briefers are available to open and close flight plans, accept PIREPs and provide basic weather services.
The picture below shows that you can talk with McAlester FSS on 122.1 and receive their response over the VOR 115.2.

EFAS/Flight Watch service on 122.0 has been discontinued, these services are now provided on the standard Flight Service frequencies.


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How to determine the MAP for an RNAV MDA approach?

On this RNAV approach plate, what is the missed approach point for the LNAV MDA approach? How do you arrive at this answer?

The Missed Approach Point (MAPt) is at the runway threshold. The other reference you will see on this RNAV chart is the Visual Descent Point (VDP), at 1.1NM RWY05R, which is at the Minimum Descent Altitude (MDA).

The concept of VDP was developed by the FAA to encourage pilots to decide to initiate a missed approach prior to reaching the MAP, in a situation where the runway or its environment is not visible at a normal descent angle. Conversely, if the runway is visible at the VDP, the pilot may continue descent, following a standard descent angle to the runway, while being assured terrain and obstacle clearance.[5] The VDP is always located prior to reaching the MAP, and is a more useful checkpoint for making the decision whether to continue on the approach or to go around than the MAP itself.

(Source: WikiPedia)
If the MAPt is intended to be elsewhere, it will be assigned its own identifier.

"The missed approach waypoint is assigned a pronounceable name when it is not located at the runway threshold."

(Source: AIM 5-4-5.d.5(d))

(Source: www.faa.gov)

How would they mark the vertical profile on a precision/non-precision approach chart if the MAP for the non-precision approach were not at the runway threshold and was not coincident with the MAP for the precision approach?   I assume the start of the dotted line would signify the MAP for the precision approach, but how would they mark the MAP for the non-precision approach (when it's not at the runway threshold)?

I would add that the visual descent point is also there to make sure you don't go below the MDA short of reaching the VDP, even if you do see the runway environment. This is to keep you from descending into obstacles short of the VDP. Suppose you are coming in at night for instance.


text:
What weather products do you use to determine whether or not a flight is safe?
There are many weather products available on AviationWeather.gov and many other sites. What do you use for preflight planning?

This is a duplicate of several questions -- See Also: What weather sources are approved by the NWS ; Where can I get additional weather information for flight planning ; What FAA weather data is available on icing

(If there is something you need to know that's not covered in those questions please try editing your question to be more specific)


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What are the types of class E airspace?
I don't quite understand the various categories of Class E airspace.

Which country or regulations are you asking about? And giving examples of the categories would help to understand your question better.

Check out this publication from the FAA

Class E airspace extends
  upward from either the surface or a designated altitude to the overlying or adjacent controlled airspace. When designated
  as a surface area, the airspace is configured to contain all
  instrument procedures. Also in this class are federal airways,
  airspace beginning at either 700 or 1,200 feet above ground
  level (AGL) used to transition to and from the terminal or
  en route environment, and en route domestic and offshore
  airspace areas designated below 18,000 feet MSL. Unless
  designated at a lower altitude, Class E airspace begins at
  14,500 MSL over the United States, including that airspace
  overlying the waters within 12 NM of the coast of the 48
  contiguous states and Alaska, up to but not including 18,000
  feet MSL, and the airspace above FL 600.

This image may help to visualize it

(source)
In more general terms its the airspace that covers most of the US below 18,000ft. Its the area where a lot of the General Aviation traffic lives (although some of it is making its way into the flight levels these days)
Side note: Depending on how you look at it class E airspace is bound to an upper limit of either 400,000 feet (76mi) which is what NASA considers re-entry altitude or what The Fédération Aéronautique Internationale defines as space starting at 62 miles so either one of those can be used as an upper bound for Class E (although officially the FAA does not define an upper bound). Re-entry point is where air drag comes into play so you would be hard pressed to get a plane over that altitude. Practically speaking the jet altitude record is held by the SR-71 Blackbird at 85,069ft and the Concorde pulls in at 60,039 ft for commercial flight.

Looks like we answered at about hte same time with the same info.

Haha i saw your answer right after I hit post.

Class E Airspace
  If the airspace is not Class A, B, C, or D, and is controlled
  airspace, then it is Class E airspace. Class E airspace extends
  upward from either the surface or a designated altitude to the
  14-3
  Figure 14-2.
  An example of a prohibited area is Crawford, Texas.
  overlying or adjacent controlled airspace. When designated
  as a surface area, the airspace is configured to contain all
  instrument procedures. Also in this class are federal airways,
  airspace beginning at either 700 or 1,200 feet above ground
  level (AGL) used to transition to and from the terminal or
  en route environment, and en route domestic and offshore
  airspace areas designated below 18,000 feet MSL. Unless
  designated at a lower altitude, Class E airspace begins at
  14,500 MSL over the United States, including that airspace
  overlying the waters within 12 NM of the coast of the 48
  contiguous states and Alaska, up to but not including 18,000
  feet MSL, and the airspace above FL 600.

You can find more about it here: https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/pilot_handbook/media/PHAK%20-%20Chapter%2014.pdf


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What type of airspace is an Atlantic Low Control Area?
Could someone explain what the blue zipper line 'Atlantic Low Control Area' is in the images below?
What type of airspace is this? How far off the coast does it start? Would it still be G to 1200 and E to 18000 in this area? Is this related to a Warning area which begins 12 NM off the coast? Why are there two parallel lines? I'm not really sure what questions to ask about it but I want to understand it well enough to be able to explain it in a check ride.

The Atlantic Low Control Area is a Class E area in international airspace, where domestic ATC procedures may be used for separation procedures.
Here are the relevant excerpts from FAA Chapter 19. Other Airspace Areas
Section 1. General

Offshore/Control Airspace Areas are locations designated in international airspace (between the U.S. 12-mile territorial limit and the CTA/FIR boundary, and within areas of domestic radio navigational signal or ATC radar coverage) wherein domestic ATC procedures may be used for separation purposes.

and

Class A Offshore/Control Airspace Areas are identified as “High" (e.g., Atlantic High; Control 1154H). Class E areas are identified as “Low" (e.g., Gulf of Mexico Low, Control 1141L)

The two parallel lines differentiate floors of class E airspace that is neither 700 feet nor 1200 feet AGL (Above Ground Level).

About the jagged lines from http://www.faa.gov/air_traffic/flight_info/aeronav/digital_products/aero_guide/:

Class E Airspace exists at 1200' AGL unless designated otherwise. The lateral and vertical limits of all Class E, (up to, but not including 18,000') are shown by narrow bands of vignette on Sectionals and TACs.
Controlled airspace floors of 700' above the ground are defined by a magenta vignette; floors other than 700' that laterally abut uncontrolled airspace (Class G) are defined by a blue vignette; differing floors greater than 700' above the ground are annotated by a symbol and a number indicating the floor.
If the ceiling is less than 18,000' MSL, the value (preceded by the word "ceiling") is shown along the limits of the controlled airspace. These limits are shown with the same symbol indicated above.

A little more search say that it is the floor altitude of that control area. The parallel lines are when the floor altitude changes (from 1300 MSL to 2700MSL and 1700MSL to 5500MSL).

So the MSL altitudes are the height at which Class E starts below which would be class G? Why is 1300 MSL written twice?

@jskypilot looking at what the webpages and legends say, it looks like they would be Class G. I don't see anything why the 1300MSL is written twice. It might be a not-so-serious map messup.


text:
Are solid-state attitude indicators subject to gimbal lock?
With all the shiny new glass cockpits it would seem that the days of the spinning mechanical gyro (and associated tumbling due to gimbal lock) should be over: Sparing everyone the boring math it should suffice to say that solid-state gyros can be engineered and built in such a way that gimbal lock is impossible, but I'm not certain that's how they're actually designed.
Do modern AHRS systems with solid-state gyros (or replacement electronic horizons like the RC Allen 2600 series) still suffer from gimbal lock, or do they provide true 3-dimensional freedom?
I'm interested primarily in answers from a light General Aviation standpoint, but answers about electronic gyros on commuter and transport category aircraft would be interesting too.

AHRS cannot suffer from gimbal lock (as there are no gimbals that can share axes):

Conventional gyros are also susceptible to gimbal lock under certain conditions. The AHRS is an all-attitude system and is free from such problems. — from A Layman's Guide to AHRS

That said, AHRS systems can still be tumbled - I know that for a while an air show routine was flown in a (then) Columbia 400 with G1000; the pilot remarked that the system Xed out only briefly while executing inverted maneuvers. In comparison, the Avidyne AHRS at the time required a full 2+ minutes of straight and level flight before resetting itself.

The G1000 has built-in attitude limitations beyond which it will not read correctly. I will try to look them up next time I'm near a technical manual. But like you said, since they can't tumble it's entirely possible to make an AHRS capable of accuracy at any attitude.

Yeah, I don't recall what they are either - I'll try to take a look at the manual I've got at home this evening.

Specific attitude limitations would lead me to believe they're using X/Y/Z (Euler) coordinates directly instead of deriving them (so "It's not gimbal locked" (because there are no gimbals), but the effect is largely the same (big red X on the attitude display when you exceed the limitations until the gyro can be reset))

Hm, I still don't understand how they get the limits. The gyros are always orthogonal, so the input has the necessary information at any attitude. Rotating the accumulated vectors is possible in any representation given correct formulas and with incorrect formulas (e.g. just adding the pitch axis to the inclination directly) will noticeably loose precision even at shallow bank angles. So it seems strange to me they could get away with incorrect formulas (and the "correct" formulas are not that complicated anyway).

@JanHudec the G1000 AHRS also uses the magnetometer for some information (it's part of why the in-flight realign takes so little time) so it's possible that outside of a certain realm the data isn't verifiable between the sensors. A dual AHRS system might be able to compare the two, but what most GA aircraft have maybe cannot?

Yes and no.  Gimbal lock is a physical AND mathematical phenomenon.  Physically, gimbals sharing the same axis can cause a lock, and mathematically, a representation of a multi-axis rotation can be in a condition which is singular, depending on how it is represented and calculated internally.
Normally, we discuss and think about aircraft attitude intuitively as set of angles: roll, pitch and heading.  In mathematical circles, this is known as a set or 3-2-1 Euler angles.  If you look into the boring (or exciting, if you are that kind of person) math, you can see a vector-matrix equation relating the change in Euler angles given a set of angular rates given in the frame of the aircraft, by common convention these are referred to as p, q and r for roll rate, pitch rate, and yaw rate respectively.  The way these rates affect the change in the angles depends on the current state of the the angles, e.g. when the aircraft is pitched, a body-frame yaw rate will alter the pitch angle.  Mathematically, this means that the "state matrix" is state-dependent and must be constantly updated using the freshest set of states available.  Any set of Euler angles will have a set of states which causes the state matrix to become poorly conditioned, resulting in numerical instability at those conditions.  For the roll-pitch-heading set of angles, this happens when pitch is + or - 90 degrees.
The way that we escape the singularities when writing the software for an AHRS is by representing the attitude in a non-singular form.  This done by using more numbers to create the representation, then adding constraints which preserve uniqueness.  A 3x3 direction cosine matrix (DCM) can do this,  It has constraints which cause it's eigenvalues to lie on a unit circle in the complex plane.  My favourite representation, however, is a quaternion constrained to have unit length.  The quaternion can be thought of in terms of Euler's rotation theorem.  There's that Euler guy again, he must have been some kind of genius.  Anyhow, the idea is that you can define an axis of rotation with three numbers, and use a fourth to represent an angle about that axis to rotate.  This gives you a system free of "mathematical gimbal lock"  The state matrix remains adequately conditioned under all states.  All you need to do to keep things friendly is to normalize the quaternion from time to time, and you always have a "background" state that you can translate into aviator-friendly Euler angles.

Does this "gimbal lock-less" system manifest on the instrument display in a glitchy way as it transits/avoids these singularities, a-la the black cat passing by in The Matrix. i.e. "Deja vu is a glitch in the matrix. It happens when they change something."?

@radarbob, no, because there are no transitions involved in avoiding the singularities. The system simply always uses a more complicated representation that does not have them.


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How much does a D-check cost compared to a new aircraft?
As I understand from those videos, a D-check basically consists in disassembling the whole aircraft, inspecting every little piece, and then reassembling it. For me, the last part seems to be more or less the job of aircraft builders, and thus its costs may be comparable to the lasts steps of building an aircraft. Thus the cost of a few D-checks may be the same as building an aircraft.
Counted in D-checks cost, how many D-checks does a newly built aircraft cost?
Let's focus on airliners. If needed, the question may be narrowed down to best sellers (lets say B737 and A320).

Keep in mind that during a D check, the parts on the plane are already owned. There will of course be a certain amount of parts replaced, but it will be much less than the manufacturer's cost to buy/manufacture all of the parts. There will be thousands of hours of labor, which will be most of the cost of a D check. The cost of maintenance space and taking the plane out of revenue service may also be considered.
According to this financial reference:

For a 15-year old Boeing 737-300, a D check would likely cost \$1.25-\$1.5 million, representing about 8-10% of the cost of the aircraft.

This book is now 15 years old along with the newest 737-300's, so in today's dollars this would be \$1.73-\$2.08 million, not accounting for changes in the cost of maintenance. Larger aircraft will have larger costs. This seems to put the cost in the same range as other sources.


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What are the main sensor value that cause High pressure compressor fault in turbofan engine?
I have read the paper "Damage Propagation Modeling for Aircraft Engine Run-to-Failure Simulation" (Reference).
 I am doing analysis to find which sensors to monitor for the faults injected in the simulated datasets.
One such fault is high pressure compressor which caused the engines to be marked as failure.
My analysis gave me the following sensors:

Corrected fan speed (NRf in rpm),
Physical fan speed (Nf rpm), and
Engine pressure ratio (P50/P2).

I could not find the proper answers online. Could anyone be kind enough to ascertain how related are these sensors for the occurrence of HPC fault in a turbofan engine.
I'm not from the aviation background. I greatly appreciate any valid suggestions/references or answers.

Hi, what type of fault do you consider ? They can be of multiple types: stall, overspeed, underspeed, vibration levels ...

@BambOo, In the document, it says that to simulate HPC degradation HPC flow and efficiency modifiers were used. The data outputs Response surface of HPC stall margin as a function of efficiency and flow losses simulating degradation in HPC module.Health
index is determined by increasing EGT (decreasing EGT
margin) as compared to HPC stall margin for all three
degradation trajectories. Each degradation trajectory was
simulated until the health index reached zero. Sensed margins (fan, HPC, LPC and EGT) were used to compute health index
to determine simulation stopping criteria.

It sounds like you are looking for an understanding of how the HPC efficiency affects the overall engine performance.  My suggestion here would be use NASA's excellent engine simulator: https://www.grc.nasa.gov/www/k-12/airplane/ngnsim.html  One of the many variables that you can play around with is HPC efficiency.  Note that as you decrease HPC efficiency, overall EPR is going to drop.  In a real engine, the controller will notice this and dump in more fuel to compensate and get back to the original EPR.  In this simulator, you will have to do that by adjusting the throttle controller.
Here's a screenshot of the engine simulator running.  The one on the left has a high compressor efficiency and the one on the right has a low compressor efficiency (note how i've changed the throttles and the different fuel flow).  Note how the temperatures on the right are all higher.  This a symptom of a less efficient component.  The engine has to work harder to generate the same thrust.  There are a number of other outputs.  You can also play around with other components, e.g. see how a degraded turbine efficiency looks versus a degraded compressor efficiency.
The only thing that this simulator does not have is rotor speed. For that, although I've not run out the calculations, I think that fan speed won't change too much with HPC efficiency, at least for a high bypass configuration.  Like I said, the control is going to change fuel flow to get back to the same thrust (via EPR), and since the majority of the thrust comes from the fan, the fan speed will be about the same.  The core speed, on the other hand, should go up.  Because the core has to work harder to produce the same amount of energy output.

I read your reference a bit, and it seems that the goal was to model degradation without looking to deep in the underlying phenomena. So basically you are just interested in the efficieny of the HPC module.
The loss of performances and therefore the wear of the module can be assessed through different probes, but the main value is the pressure ratio between the inlet and the outlet of the HPC module.
This ratio will highly depend on multiple wear phenomena occuring inside the module, especially the wear of the structures due to rotor to stator rubs. Also, small damages due to fatigue failures may lead to pressure ratio losses.

Thank you for having a glance at the paper. I am trying to understand if there is any way the corrected fan speed, physical fan speed and energy pressure ratio effects HPC ?

@BambOo.  Your answer is not wrong - but there's a bit more too it. First, the pressure ratio you mention, isn't always measured in the standard instrumentation set of an engine (you need the total pressures, not static, which is what is normally measured). Second, the ratio changes with corrected rotor speed. So you also have to measure N2, and the engine inlet total pressure and temperature to give corrected N2. Thirdly, efficiency is defined for a compressor as the actual temperature rise, compared to the isentropic temperature rise, that should occur for the given pressure rise.

...So, you also have to measure the total temperature at the inlet and outlet of the compressor (which I've never seen in a production engine). But, if you do have all these measurements, you should be able to detect a change in compressor efficiency, if it's big enough above the noise level.

@Joe. It's the other way around. HPC efficiency affect these parameters (and others ) not vice-a-verse. Daniel's answer gives a good overview. The details depend on what the engine control is trying to do.  (See https://aviation.stackexchange.com/questions/32301/what-is-the-difference-between-epr-and-rotor-speed-as-thrust-setting-parameter). In simple summary, a GE engine will adjust fuel flow to keep corrected N1 constant, a PW or RR engine will adjust fuel flow to keep EPR constant. In both cases, the fuel flow will go up, as the HPC degrades, but not necessarily by the same amount.

...All other parameters will except EPR, or corrected N1, will shift, because every thing is thermodynamically linked.

Very interesting remarks @Penguin. I have to admit that I come from the modellng / mechanics community rather than the actual piloting or more practical community. I totally agree with the fact that the controller will try to keep performances whatever the situation...

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
If the APU is switched off but the APU bleed is on, will the engine bleed valve close automatically?
On he A320 family aircraft, if the APU is switched off but the APU bleed is on, will the engine bleed valve closed by the Bleed Monitoring Computer (BMC)?
This what I understood after going through the APU chapter of the FCOM. Just wanted to confirm because the way I understand it is that the APU bleed valves will not open since it requires a minimum of 8 PSI, so the engine bleed valve should be open.

No . Here I am not copy pasting the question but this what I thought after going through the APU chapter of the FCOM. Just wanted to know the correct answer for the same because according to me if APU is off even with APU bleed switch ON the APU bleed valves will not open since It requires a minimum of 8 PSI so engine bleed valve should be open according to me

@SourabhJain, I've tried to include the text from your comment into the question. Please check if I did represent your intention correctly.

The answer is in the FCOM:

So, the ENG bleed valve closes if:

Reverse flow detected
ENG 1 starter valve not closed
APU bleed valve open
Bad things (Overpressure / Overheat / Leak)
ENG BLEED switch is off or FIRE pb pushed

So, if APU is not ON, the APU BLEED valve is closed, and then the condition 3 is false. If all the others conditions are also false, then the ENG valve would open.


text:
Is it possible to build a flight-capable aircraft powered by compressed gas?
I pray this isn't a physics SE question.
A star engine can be considered "gas operated". So what if we power a star engine with unleashed, formerly compressed gas instead of... actual gas/aero-kerosene/etc...? So instead of fuel tank (oh the temptation to pun is so strong here) you use gas tank?
Note that since nothing has to BURN, we can use lighter-than-metal material, as engines usually play a big part when it comes to weight of aircraft.
Can you?
And if that's a YES, can you estimate the flight time?

No, fuel plays a much larger part when it comes to weight of the aircraft.

I don't understand your question. Are you asking if it would be possible to burn a gaseous fuel instead of liquid kerosene? Are you asking if it would be possible to burn helium? (No, it's completely unreactive.) Something else?

I'm guessing the question is about using compressed gas as an energy source, as in a pneumatically driven propeller.

@AEhere gets it right.

Yes, but only on extremely small planes (handheld models).  The flight time will be incredibly short, and the design will not scale linearly.  I'd be surprised if you could ever get the wingspan much above 10 feet.

In aircraft design, the main problem is energy density of the energy source. Even with the phenomenally high energy density of kerosene fuels (>42 MJ/kg), the fuel still represents a sizable chunk of the weight of an aircraft. For example, fueling an Airbus A330 will double its weight!
According to Wikipedia, compressed air has a practical energy density of about 0.1MJ/kg. There is no way you can offset the enormous amount of compressed air required by downsizing your engines. You would go from 110.000 kg of fuel to over 400 million kg of compressed air.
A more relatable example would be a Cessna 172. It can carry 212 litres of fuel, or about 150kg. On top of that, it can carry about 190kg of useful payload. If we instead use compressed gas, we would need over 60.000kg of compressed gas to get the same range.

It's not even the weight of the compressed air itself that's going to be your major problem.  It's the weight of the tanks needed to hold it.  For instance, an empty scuba tank weighs about 35 lbs, a full one (at ~3000 PSI) only about 40 lbs.  So that 400 million kg of compressed air might need a couple of billion kg worth of tanks to hold it.

@jamesqf I don't know if the quoted energy density includes air tanks. Regardless, it's so far off that implementation details won't make a difference.

Yes
Whereas it may not be feasible, as Sanchises explained, for large manned airplanes, on the scale of small models it is quite possible. These are aircraft, too!

(source)
Such models were quite popular in the '60s and '80s: the engine is very simple indeed, and can be powered by the common CO2 cartridge (as used for making fizzy water at home, which was also more popular at that time).
Nowadays, such engines are largely replaced by electric drive: also something not-yet-quite-working on a large scale, but excellent for small models.

Don't forget Airhogs -- model airplanes (and other, ground-bound models, too) powered by low pressure compressed air (hand pumped in a plastic pressure tank) driving a nearly identical motor made of plastic.  Flight times of up to about a minute, if everything was just right...

If you allow to store the gas in liquefied form, the answer is a cautious Yes. But do not expect more than very brief flights.
Not compressed, but liquefied gas was used in the first attempts at powered flight. Before internal combustion engines became light and fast enough for use in aircraft, carbonic acid engines were employed by several pioneers.
Otto Lilienthal, who built and flew the first man-carrying gliders, added carbonic acid engines of his own design to several of them. They drove flapping wingtips which, according to his own description, clearly helped in stretching the glide, but were not powerful enough for sustained flight.

Lilienthal glider with flapping wingtips (picture source; a picture of Lilienthal's engine can also be found on that page)
In 1905, the Romanian engineer Trajan Vuja used a carbonic acid engine to fly his high-wing monoplane. It was powered by a 25 hp modified Serpollet engine which drove a tractor propeller. It was capable of short hops only, even though Wikipedia reports that

The fuel supply was enough for a running time of about five minutes at full power

In its "ten years ago" section, the October 19, 1916, issue of Flight International described the power plant of the Vuja machine as:

A peculiarity of the arrangement is that the motor is driven by liquid carbonic acid, a method of obtaining power which has not hitherto been conspicuous for its lightness.

Of course, carbon-based propellants are unacceptable today, while other heavy gases are either pollutants too, or scarce and expensive (a tankful of xenon anybody?). A metal hydride can store a surprising quantity of gas, but hydrogen is too light to exert thrust efficiently. Compressed air is about your best bet.
Frenchman Victor Tatin flew a 1.9 m (6 ft 3 in) span model driven by compressed air in 1879. The air engine powered two propellers. It was tethered and flew in a circle, for a distance of around 15 m (50 ft), after taking off under its own power. It is now in a French museum: see photo.

There is also a compressed-air model aeroplane from the early pioneer days, on display in the National Aerospace Library, Farnborough, UK. It was reasonably successful but I can't recall who made it. Anybody?

Can you expand on your criteria for "reasonably successful"?  Because rubber bands are reasonably successful at powering balsa models, yet nobody in their right mind would consider them for general aviation...

@MichaelHall Not unless somebody can recall whose model it was, sorry. And I agree that rubber power is a fair analogy here.


text:
What is the difference between ICAO and non-ICAO flight address?
In document 260B, "Minimum operational performance standards for 1090Mhz Extended Squitter", on page 47 of the pdf, in the DF18 format they have mentioned for a particular CF(code format), "non-ICAO flight address".
So I wanted to know what the difference was between ICAO and non-ICAO flight address?

Hello @vinas sunder, welcome to aviation.stackexchange.com.

The use of the non-ICAO flight address field is described in the very same document, on table 2-11, page 53 of the PDF:

AA field holds another kind of address for the transmitting
  ADS-B Participant: a self-assigned "anonymous" address, a
  ground vehicle address, or a surface obstruction address.

Ground vehicles and surface or tethered obstructions want to participate in ADS-B, but they don't have ICAO aircraft addresses, so they have to have their own addresses. Since these addresses aren't centrally assigned, the CF field is used to keep them distinct: otherwise, you could have a vehicle address that conflicts with an ICAO aircraft address.
The different kinds of addresses are further described in section 2.2.5.1.1 (p 172 of the PDF).


text:
How to get IATA code for the Airline PSS system?
If we are having airline PSS (passenger Service system) System, how can we registered with IATA and get IATA code. as an example Amadeus is using 1A, Sabre is using 1S, SITA is using XS etc..

Have you tried asking IATA?

any contact details

Finding the contact section on iata.org takes about one point seven seconds (on my phone on mobile data). I‘m sure you‘ll manage!

Try below link, you can type PSS system name and get the code for the same. However, not all PSS are IATA registered.
https://www.iata.org/en/publications/directories/code-search/


text:
How could so many drones establish contact with ATC?
India has brought out best set of drone regulations on the basis of No Permission No Take-off (NPNT).
But how can the ATC establish contact with so many  drones that will be unpredictably asking for take off.
Also how can they be monitered through a controlled airspace? Will it be miNiaturr ADSBs fixed or something else?

"best set of drone regulations" - depends on whether your the pilot of an aircraft or a drone.

I'm not sure about everywhere but in the U.S. there is an app called "Airmap" Link to webpage. It's built for commercial drone operators but it can also be used by people flying for fun.
There is a flight plan portion of the app that allows you to file for a flight. You specify time of takeoff, radius you'll fly in, altitude and aircraft. This is then submitted for approval from the FAA.
It takes a matter of seconds and your approved, or not depending on the situation. It also has a map portion that displays the other approved flight plans in the area, if the pilot made his public, so you could even use this to join other's flying at the park if you see a flight pop up or something like that.
This could be a viable answer for India as well, coupled with a mini ADS-B, like CrossRoads mentioned, could prove even better.

ADS-B Out units are getting pretty small for reporting position.
Example from uAvionix.com: https://uavionix.com/products/ping1090/
Requesting takeoff clearance could be done via an automated system, where the operator enters takeoff time, time enroute, planned route, etc.  Enter it some minimum number hours ahead of time for review and approval.

I am not familiar with the Indian system, however, it may be useful to discuss the US system, which is fundamental for future development.
First, the FAA has instituted a UAS Data Exchange provides for data exchange between FAA and industry partners.  LAANC  (Low Altitude Authorization and Notification Capability) provides near real-time data exchange.  Private industry partners, there are several approved thus far, have interfaces to users, which accept the user's flight plan, and routes it to ATC for authorization.  Made more efficient by predetermined grid maps near airports, etc. the FAA can rapidly approve requests without substantial interference determinations. (Table lookups, versus separation calculations.)  The industry partner routes the approval back to the end user.
In addition, the industry partner makes available WX, NOTAMS, etc. through a user app.
If the operation of this system were modified, it is conceivable that there could be a UAS hold until a clearance is received.  Fortunately, that has not happened.  UAS are utilized at locations where there is limited communications (cellphone data) availability, and that lack of communications would impair UAS operations in those areas. And areas away from airports and some airspace will not require the explicit coordination that LAANC offers. In urban areas, or more specifically, areas near airports and restricted airspace, there is greater availability of communication, which makes LAANC more easily implementable.
At this time, LAANC covers about 500 US airports, and there is a manual process for areas not covered.  There are about 14 industry providers, although only a few effectively offer services to individuals.  An example is AirMap.
With respect to the issue of utilizing ADSB and similar technologies, while that may be useful, one objective of UAS  traffic management is to segregate UAS and conventional aircraft.  The enforcement of that segregation could be affected by utilizing downloadable, per flight, boundaries to the UAS, so that it is effectively rendered incapable of flying outside a clearance limit.
Real-time ATC monitoring of ADSB would cause capacity issues, as well as impose communications requirements to the UAS pilot.  A far easier approach is to pre-clear operations and relies on the UAS to comply with an advance clearance, as well as the intrinsic separation of UAS and conventional aircraft.
Hopefully, this provides some insight as to how a prior to taking off system could be implemented, leveraging existing FAA facilities.  I do not know any specifics of the implementation of regulations for India.


text:
How do you calculate total drag on a wing?
I'm not an aeronautical engineer, but I am making a brave attempt to perform a preliminary design for an 4-metre wingspan radio controlled glider. With special thanks to @mns I have now learned a formula for calculating the coefficient of induced drag. The question now is how do I use it to calculate total drag.
Considering the formula below, how do I make a reasonable estimation of efficiency factor (e)

I've selected a prismatic inner wing with tapered outer wing to assimilate an ellipse? I understand that they state that (e) for a perfect ellipse is 1. What is 0 then, a square? Here is my wing planform:

They then state that I should add CDi to CD0 to offer me a total CD.
The question I ask now is why add CD0 (That is CD at 0 degrees AoA if my angle of incidence (setting angle of the wing) is 6.25 degrees?
I assume that I then run this through the lift formula to get drag in newtons.
The curve for CD / Alpha can be seen here

Typical values of e are between 0.7 and 0.98 in subsonic flow, and 0.3 to 0.5 in supersonic flow. Since it is in the denominator, e cannot become 0 or drag would be infinite. A square wing has an e anywhere between 1 (with aspect ratio close to zero) and 0.8 (with high aspect ratio and away from the design point for the twist distribution)
As a rough first-order assumption, the zero-lift drag coefficient is constant over the moderate AoA range when the flow is attached over the full chord. This drag is the sum of friction (shear forces tangential to the wing surface) and pressure drag (caused by pressure on the backwards-facing sections being lower than on the forward-facing sections of the airfoil, again due to viscosity).
Of course, once you have a full airfoil polar, your results will be more precise if you use the particular value of the drag coefficient at the specified angle of attack. However, sometimes you will need to calculate lift and drag without knowing the angle of attack, and then you fall back to the first-order approximation. For an example where this assumption is useful please read this answer.

Problem is first approx is wrong, because at higher AoA pressure drag is higher(even lower pressures at backward facing sections). I never understand why theory neglect pressure drag as AoA rise, and keep zero lift pressure drag through all AoA range.. If increase AoA of hand  out of car window, you feel huge increase in drag/resistance, is this "pressure drag" or "induced drag"?

@user207141 No, this approximation is useful and - as I said - something to fall back to if no other data is available. Your example with the hand is pressure drag from flow separation. There you are outside of the linear AOA range and that area is of no interest for flight performance.

I just say pressure drag when wing produce  lift is higher then on zero lift AoA, so total drag must be higher than formula predict... In other words formula is too simplified..

@user207141 Look here for a counterexample.